This invention generally relates to plastic containers for retaining a commodity, and in particular a liquid commodity. More specifically, this invention relates to a plastic container base structure that allows for significant absorption of vacuum pressures by the base without unwanted deformation in other portions of the container.
Numerous commodities previously supplied in glass containers are now being supplied in plastic, more specifically polyester and even more specifically polyethylene terephthalate (PET) containers. The manufacturers and fillers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable, and manufacturable in large quantities.
Manufacturers currently supply PET containers for various liquid commodities, such as beverages. Often these liquid products, such as juices and isotonics, are filled into the containers while the liquid product is at an elevated temperature, typically 68xc2x0 C.-96xc2x0 C. (155xc2x0 F.-205xc2x0 F.) and usually about 85xc2x0 C. (185xc2x0 F.). When packaged in this manner, the hot temperature of the liquid commodity is used to sterilize the container at the time of filling. This process and the containers designed to withstand it are respectively known as hot filling, and hot fill or heat set containers.
Hot filling works, and is an acceptable process, with commodities having a high acid content. Non-high acid content commodities, however, must be processed in a different manner. Nonetheless, manufacturers and fillers of non-high acid content commodities desire to supply PET containers for these commodities as well.
For non-high acid commodities, pasteurization and retort are the preferred sterilization methods. Pasteurization and retort both present an enormous challenge for manufactures of PET containers in that heat set containers cannot withstand the temperature and time demands of pasteurization and retort.
Pasteurization and retort are both methods for cooking or sterilizing the contents of a container after it has been filled. Both processes include the heating of the contents of the container to a specified temperature, usually above about 70xc2x0 C. (about 155xc2x0 F.), for a specified length of time (20-60 minutes). Retort differs from pasteurization in that higher temperatures are used, as is an application of pressure externally to the container. The pressure is necessary because a hot water bath is often used and the overpressure keeps the water, as well as liquid in the product, in liquid form above its boiling point temperature.
The present invention will find particular utility in hot fill applications, vacuum seal applications and applications where water loss through the container is a concern. It may also find utility in pasteurization and retort applications.
PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to maintain its material integrity is related to the percentage of the PET container in crystalline form, also known as the xe2x80x9ccrystallinityxe2x80x9d of the PET container. Crystallinity is characterized as a volume fraction by the equation:   Crystallinity  =            ρ      -              ρ        a                            ρ        c            -              ρ        a            
where xcfx81 is the density of the PET material; xcfx81a is the density of pure amorphous PET material (1.333 g/cc); and xcfx81c is the density of pure crystalline material (1.455 g/cc).
The crystallinity of a PET container can be increased by mechanical processing and by thermal processing.
Mechanical processing involves orienting the amorphous material to achieve strain hardening. This processing commonly involves stretching a PET container along a longitudinal axis and expanding the PET container along a transverse or radial axis. The combination promotes what is known as biaxial orientation in the container. Manufacturers of PET bottles currently use mechanical processing to produce PET bottles having about 20% crystallinity in the container""s sidewall.
Thermal processing involves heating the material (either amorphous or semi-crystalline) to promote crystal growth. On amorphous material, thermal processing of PET material results in a spherulitic morphology that interferes with the transmission of light. In other words, the resulting crystalline material is opaque (and generally undesirable). Used after mechanical processing, however, thermal processing results in higher crystallinity and excellent clarity. The thermal processing of an oriented PET container, which is known as heat setting, typically includes blow molding a PET preform against a mold heat to a temperature of about 120xc2x0 C.-130xc2x0 C. (about 100xc2x0 F.-105xc2x0 F.), and holding the blown container for about 3 seconds. Manufacturers of PET juice bottles, which must be hot filled at about 85xc2x0 C., currently use heat setting to produce PET bottles having a crystallinity range of 25-30%.
After being hot filled, the heat set containers are capped and allowed to reside at generally about the filling temperature for approximately five minutes. The container along with the product is then actively cooled so that the container may be transferred to labeling, packaging and shipping operations. Upon cooling, the volume of the liquid in the container is reduced. This reduction in volume results in the creation of a vacuum within the container. Generally, vacuum pressures within the container range from 1-300 mm/Hg. If not controlled or otherwise accommodated, these vacuum pressures result in deformation of the container which leads to either an aesthetically unacceptable container or one which is unstable. Typically, vacuum pressures have been accommodated by the incorporation of structures in the sidewall of the container. These structures are commonly known as vacuum panels. Vacuum panels are designed to distort inwardly under the vacuum pressures in a controlled manner so as to eliminate undesirable deformation in the sidewall of the container.
While vacuum panels have allowed the containers to withstand the rigors of a hot fill procedure, they do present some limitations and drawbacks. First, during labeling, a wrap-around or sleeve label is applied to the container over the vacuum panels. Often, the appearance of these labels over the sidewall and vacuum panels is such that the label is wrinkled and not smooth. Additionally, when grasping the container, the vacuum panels are felt beneath the label resulting in the label being pushed into the various crevasses and recesses of the vacuum panels.
It would therefore be desirable to have a container which could accommodate the vacuum pressures which result from hot filling yet which has or is capable of having smooth sidewalls.
In view of the above, it is an object of the present invention to provide a plastic container which principally accommodates vacuum pressure through a mechanism other than vacuum panels in the sidewalls of the container.
A further object of the present invention is to provide a container having a base structure which accommodates vacuum pressure while preventing undesirable distortion in other parts of the container.
Still another object of this invention is to provide a plastic container in which the base structure is substantially flat in cross-section in a wall portion thereof which cooperates with an upstanding shoulder wall or ridge to permit the accommodation of vacuum pressures within the base structure.
Accordingly, this invention provides for a plastic container which maintains aesthetic and mechanical integrity during any subsequent handling after being hot filled and cooled to ambient.
Briefly, the plastic container of the invention includes an upper portion, a body or sidewall portion and a base. The upper portion includes an opening defining the mouth of the container, a threaded portion (or other configuration) as a means to engage a closure, and a support ring that is used during handling, before, during, and after manufacturing. The upper portion further includes a shoulder extending down to the sidewall portion which generally defines the greatest diameter of the container.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.